1. Field of the Invention
The invention relates to a liquid crystal composition suitable for use in an active matrix (AM) element, and an AM element including the composition. In particular, the invention relates mainly to a composition having a nematic phase and a composition having a positive dielectric anisotropy.
2. Related Art
On a liquid crystal display element, classification based on an operating mode of liquid crystals includes phase change (PC), twisted nematic (TN), super twisted nematic (STN), electrically controlled birefringence (ECB), optically compensated bend (OCB), in-plane switching (IPS), vertical alignment and so forth. Classification based on a driving mode includes a passive matrix (PM) and an active matrix (AM). PM is further classified into static, multiplex and so forth, and AM is classified into a thin film transistor (TFT), a metal insular metal (MIM) and so forth. TFT is further classified into amorphous silicon and polycrystal silicon. The latter is classified into a high temperature type and a low temperature type according to a production process. Classification based on a light source is a reflection type utilizing a natural light, a transmission type utilizing a backlight and a semi-transmission type utilizing both the natural light and the backlight.
These elements include a liquid crystal composition having suitable characteristics. The liquid crystal composition has a nematic phase. General characteristics of the composition should be improved to obtain an AM element having good general characteristics. Table 1 below summarizes a relationship between the two general characteristics. The general characteristics of the composition will be explained further based on a commercially available AM element. A temperature range of a nematic phase relates to the temperature range in which the element can be used. A desirable range at a higher limit temperature of the nematic phase is approximately 70° C. or more and a desirable range at a lower limit temperature is approximately −20° C. or less. The viscosity of the composition relates to the response time of the element. A short response time is desirable for displaying a moving image. Accordingly, a small viscosity of the composition is desirable. A small viscosity at a low temperature is more desirable.
TABLE 1General characteristics of a liquid crystalcomposition and an AM elementGeneral characteristicsGeneral characteristicsNoof a compositionof an AM Element1Temperature range of a nematicUsable temperature range is widephase is wide2Viscosity is small1)Response time is short3Optical anisotropy is suitableContrast ratio is large4Threshold voltage is lowElectric power consumption issmall and a contrast ratio is large5Specific resistance is largeVoltage holding ratio is large anda contrast ratio is largeNote1)A liquid crystal composition can be injected into a cell in a short time.
The optical anisotropy of the composition relates to the contrast ratio of the element. A product (Δn·d) of the optical anisotropy (Δn) of the composition and the cell gap (d) of the element is designed to be approximately 0.45 micrometers to maximize the contrast ratio of the element. Accordingly, the optical anisotropy of the composition is in the range from approximately 0.08 to approximately 0.12. In recent years, Δn·d is designed to be from approximately 0.38 to approximately 0.42 micrometers to decrease the response time of the device. In this case, the optical anisotropy of the composition is in the range from approximately 0.10 to approximately 0.15. A low threshold voltage in the composition contributes to a small electric power consumption and a large contrast ratio of the element. Accordingly, a low threshold voltage is desirable. A large specific resistance of the composition contributes to a large voltage holding ratio and a large contrast ratio of the element. Accordingly, a composition having a large specific resistance not only room temperature but also high temperature in the initial stage is desirable. A composition having a large specific resistance not only room temperature but also high temperature even after it has been used for a long time is desirable.
The conventional compositions are disclosed in the following documents. JP S60-51135 A/1985 (U.S. Pat. No. 4,594,465, EP 0 132 377A2), JP H01-500860 T/1989 (WO88/02130A2), JP H01-503455 T/1989 (WO87/07890A2), JP H04-279695 A/1992, JP H09-183974 A/1997 (GB 2 300 642 A), JP H10-501019 T/1998 (WO95/33802A1), and WO 2004/035710/2004. JP H01-503455 T/1989 (WO87/07890A2), JP H04-279695 A/1992 and JP H10-501019 T/1998 (WO95/33802A1) relate to a ferroelectric liquid crystal composition (a composition containing a smectic phase). JP H01-500860 T/1989 (WO88/02130A2) and JP H09-183974 A/1997 (GB 2 300 642 A) relate to a liquid crystal composition containing a nematic phase and having a negative dielectric anisotropy. On the other hand, JP S60-51135 A/1985 (U.S. Pat. No. 4,594,465, EP 0 132 377A2) and WO 2004/035710/2004 relate to a liquid crystal composition containing a nematic phase having a positive dielectric anisotropy.
A desirable AM element has such characteristics as a large usable temperature range, a short response time and a large contrast ratio. The response time is desirably as shorter by 1 msec as possible. Therefore, a composition having such characteristics as a high higher limit temperature of the nematic phase, a low lower limit temperature of the nematic phase, a small viscosity, a suitable optical anisotropy, a low threshold voltage and a large specific resistance is desirable.